1. Field of the Invention
The present invention relates to a grid array package, and more particularly, to a grid array package with increased electrical grounding routes and a method of fabricating the same.
2. Description of the Prior Art
Integrated circuits areused in a wide variety of electrical devices such as intelligent appliances (IA), personal computers (PC), and so on. Generally speaking, integrated circuits are enclosed inside packages to protect their fragile circuitry. The packages also function to connect integrated circuits to power, to exchange data with outside circuitry, and to ground the current of the integrated circuits. Finally, the packages are also interfaces for heat dissipation, this function becoming increasingly important for modern integrated circuits as their transistor counts and operating frequencies continue to climb. Thus, dissipation of the heat generated during the operation of integrated circuits is an important issue in designing packages.
Please refer to FIG. 1. FIG. 1 is a structural schematic diagram of a four-layer ball grid array package 10 for packaging an integrated circuit 12. The package 10 comprises a substrate 14 with a top surface 11 and a bottom surface 15. The package 10 further comprises an integrated circuit 12 mounted on the top surface 11 and four conducting layers located within the substrate 14:a bonding pad layer 16, a first inner conducting layer 18, a second inner conducting layer 20 and a contact pad layer 22. A plurality of die pads 30 disposed on the integrated circuit 12 are electrically connected to bonding ring 32 and bonding fingers 34 disposed on the top surface 11 of the substrate 14 through conducting wires 40, 42 and 44. The bonding rings 32 and the bonding fingers 34 are electrically connected to via holes 26 through traces 17. The via holes 26 are electrically connected to the contact pad layer 22, which comprises a plurality of contact pads 24 formed on the bottom surface 15 of the substrate 14. In addition, the via holes 26 pass through the first inner conducting layer 18 and the second inner conducting layer 20 and may or may not make contact with those layers according to circuit design. To reduce the probability of an improper connection, a plurality of channels is formed in the first inner conducting layer 18 and the second conducting layer 20 and filled with a dielectric material 28. A plurality of solder balls 36 is adhered onto the contact pads 24 for connection with outside circuitry. A route from the integrated circuit to outside circuitry runs from the die pads 30, through the conducting wires 40, 42, 44, the bonding rings 32 and bonding fingers 34, the traces 17, thevia holes 26, the contact pads 24 and the solder balls 36. The solder balls connect the package to outside circuitry.
The route for connection with outside circuitry has to pass through the first inner conducting layer 18 for grounding,and the first inner conducting layer 18 is also called a ground plane. Likewise, the second inner conducting layer 20 is also called a power plane because it is used for connecting to the power.
Please refer to FIG. 2. FIG. 2 is a bottom view of the package 10. The solder balls 36 in FIG. 2 can be divided into three groups according to their position: a center array, an intermediate group, and an outer array. The center array is composed of ground solder balls 48 for grounding. The intermediate group of contact pads comprises power solder balls 46 for providing routes to connect the package 10 to the power. The ground solder balls 48 of the center array not only provide a route for grounding but also a route for heat dissipation. If only the outer array of contact pads were present, routes from the integrated circuit 12 to the solder balls 36 would be long, and the length of the routes would affect the efficiency of heat dissipation in the package 10. The addition of the center array of ground solder balls 48 allows redundant heat to be directly transferred to the outside circuitry.
Although the package 10 has more efficient heat dissipation, the presence of the intermediate group of contact pads causes narrow current routes. For detailed description, please refer to FIG. 3, which is a schematic diagram of the first inner conducting layer 18. The first inner conducting layer 18 is also the ground plane of the package 10. A peripheral portion 47 of the first inner conducting layer 18 comprises contact via holes 31 that electrically connect with the ground plane and intact via holes 33 that do not electrically connect with the ground plane. As discussed above, the first inner conducting layer 18 may have channels filled with dielectric materials 28 surrounding the via holes 26. Since the power solder balls 46 surround the ground solder balls 48 and the dielectric materials 28 surrounds the via holes 26, only a few narrow routes can be used to connect a central portion 49 of the ground plane with a peripheral portion 47 of the ground plane. This hindrance in current flow can lead to unexpected electro-magnetic interference and can further affect the operation of the integrated circuit 12 and efficiency of heat dissipation in the package 10.
It is an object of the claimed invention to provide a grid array package with increased grounding routes and a method of forming the same to solve the problem of grounding plane splitting.
In accordance with the claimed invention, the grid array package includes a substrate with a top surface and a bottom surface. On the bottom surface is aninner array of contact pads where two most adjacent contact pads of the inner array of contact pads are separated by a first distance, an outer array of contact pads where two most adjacent contact pads of the outer array of contact pads are separated by a second distance, and a intermediate group of contact pads with at least one ground contact pad for grounding the grid array package. The intermediate group of contact pads is located between the inner array of contact pads and the outer array of contact pads, and is separated from the inner array of contact pads by a third distance and from the outer array of contact pads by a fourth distance. The third distance and the fourth distance are greater than the first distance and the second distance.
It is an advantage of the claimed invention that the package has an increased number of grounding routes to improve the efficiency of heat dissipation.